Use lowercase names for interpreters

* So the mapping to `engine-version` versions is clearer.

Author: eregon

README.md

@@ -14,9 +14,9 @@ This action currently supports these versions of MRI, JRuby and TruffleRuby:
 
 | Interpreter | Versions |
 | ----------- | -------- |
-| Ruby | 2.1.9, 2.2, 2.3.0 - 2.3.8, 2.4.0 - 2.4.10, 2.5.0 - 2.5.8, 2.6.0 - 2.6.6, 2.7.2, head, debug, mingw, mswin |
-| JRuby | 9.1.17.0, 9.2.9.0 - 9.2.13.0, head |
-| TruffleRuby | 19.3.0 - 20.3.0, head |
+| `ruby` | 2.1.9, 2.2, 2.3.0 - 2.3.8, 2.4.0 - 2.4.10, 2.5.0 - 2.5.8, 2.6.0 - 2.6.6, 2.7.2, head, debug, mingw, mswin |
+| `jruby` | 9.1.17.0, 9.2.9.0 - 9.2.13.0, head |
+| `truffleruby` | 19.3.0 - 20.3.0, head |
 
 `ruby-debug` is the same as `ruby-head` but with assertions enabled (`-DRUBY_DEBUG=1`).  
 On Windows, `mingw` and `mswin` are `ruby-head` builds using the MSYS2/MinGW and the MSVC toolchains respectively.